Sepsis is a morbid condition induced by a toxin, the introduction or accumulation of which is most commonly caused by infection or trauma. The initial symptoms of sepsis typically include chills, profuse sweat, irregularly remittent fever, prostration and the like, followed by persistent fever, hypotension leading to shock, neutropenia, leukopenia, disseminated intravascular coagulation, adult respiratory distress syndrome and multiple organ failure.
Sepsis-inducing toxins have been found associated with pathogenic bacteria, viruses, plants and venoms. Among the well described bacterial toxins are the endotoxins or lipopolysaccharides(LPS) of the gram-negative bacteria. These molecules are glycolipids that are ubiquitous in the outer membrane of all gram-negative bacteria. While chemical structure of most of the LPS molecule is complex and diverse, a common feature is the lipid A region of LPS [Rietschel, E. Th. et al., in Handbook of Endotoxins, 1:187-214 eds. R. A. Proctor and E. Th. Rietschel, Elsevier, Amsterdam (1984)]; recognition of lipid A in biologic systems initiates many, if not all, of the pathophysiologic changes of sepsis. Because lipid A structure is highly conserved among all types of gram-negative organisms, common pathophysiologic changes characterize gram-negative sepsis.
Current concepts support the contention that the primary response of the host to LPS (including man) involves the recognition of LPS by cells of the monocyte/macrophage lineage, followed by the rapid elaboration of a variety of cell products including the general group known as cytokines. Other cell types believed to participate in sepsis and in particular in the response to LPS are polymorphonuclear leukocytes and endothelial cells; each of these cell types are also capable of responding to LPS with the elaboration of potent inflammatory substances.
LPS is believed to be a primary cause of death in humans during gram-negative sepsis, particularly when the symptoms include adult respiratory distress syndrome (ARDS). van Deventer et al., Lancet, 1:605 (1988); Ziegler et al., J. Infect. Dis., 136:19-28 (1987). For instance, one particular cytokine, tumor necrosis factor alpha/cachectin (TNF), has recently been reported to be a primary mediator of septic shock. Beutler et al., N. Eng. J. Med., 316:379 (1987). Intravenous injection of LPS endotoxin from bacteria into experimental animals and man produces a rapid, transient release of TNF. Beutler et. al., J. Immunol., 135:3972(1985). Mathison et al., J. Clin. Invest. 81:1925 (1988). Evidence that TNF is a critical mediator of septic shock comes primarily from experiments in which pretreatment of animals with anti-TNF antibodies reduces lethality. Beutler et al., Science, 229:869, (1985). Mathison et al., J. Clin. Invest. 81:1925 (1988). These reports suggest that interruption of the secretion of TNF caused by LPS or other factors would ameliorate the often lethal symptoms of sepsis.
Upon introduction of LPS into the blood, it may bind to a protein termed lipopolysaccharide binding protein (LBP). LBP is a 60 kD glycoprotein present at concentrations of less than 100 ng/ml in the serum of healthy animals and man. During the acute phase, LBP is synthesized by hepatocytes, and reaches concentrations of 30-50 ug/ml in serum. LBP can be purified from acute phase human and rabbit serum. Tobias, et al., J. Exp. Med., 164:777-793 (1986). LBP recognizes the lipid A region of LPS and forms high affinity, 1:1 stoichiometric complexes with both rough and smooth form LPS. Tobias, et al., J. Biol. Chem., 264:10867-10871 (1989). LBP bears N-terminal sequence homology with the LPS-binding protein known as bactericidal permeability-increasing factor, (BPI). Tobias, et al., J. Biol. Chem., 263:13479-13481, (1988). BPI is stored in the specific granules of PMN [Weiss, et al., Blood, 69:652-659, (1987)] and kills gram negative bacteria by binding LPS and disrupting the permeability barrier. Weiss, et al., J. Immunol., 132:3109-3115, (1984). In contrast to BPI, LBP is not directly cytotoxic for gram-negative bacteria [Tobias, et al., J. Biol. Chem., 263:13479-13481, (1988)] and its precise biological function has been obscure.
By way of further background, the cells of the monocyte/macrophage lineage perform diverse immune function including the phagocytosis of microorganisms, the uptake of antigenic material and its presentation in a form which is stimulatory to helper T cells. They are probably also involved in the immune surveillance against tumors and they secrete some complement components and cytokines. Surface membrane antigens play a critical role in regulating these activities. Several monocyte/macrophage surface antigens have been identified and their molecular weight has been determined. One such antigen, CD14, is a 55-kD glycoprotein expressed by monocytes, macrophages, and activated granulocytes. It is recognized by a number of monoclonal antibodies (mAbs) including MO2, MY4, 3C10 and LEUM3. Although no biological function has yet been ascribed to CD14, its restricted expression on mature cells suggests an important effector function. The nucleotide sequence of the gene encoding the monocyte cell surface differentiation antigen CD14 has been determined and the amino acid residue sequence of CD14 has been deduced therefrom. (Ferrero et al., Nucleic Acids Research Vol. 16:4173 (1988)